Papillomaviruses are a group of small DNA viruses that cause disease and pathological conditions in animals and humans These tumorigenic viruses produce benign tumors or lesions which may, in some instances, develop into malignancies. Papillomaviruses have been implicated as a cause of cervical cancer, as well as other anogenital and epithelial malignancies.
Papillomaviruses consist of icosahedral particles containing protein and a single, circular, double-stranded DNA molecule averaging 7.8 kbp. To date, more than ten animal papillomaviruses and more than fifty-five human papillomaviruses have been identified (R. Sousa et al., "Control of Papillomavirus Gene Expression", Biochimica et Biophysica Acta, 1032, pp. 19-37 (1990); E. M. DeVilliers, "Heterogeneity of the Human Papillomavirus Group", J. Virol., 63, pp. 4898-903 (1989)). One particularly studied papillomavirus is bovine papillomavirus ("BPV").
All known papillomaviruses encode similar proteins that perform analogous functions in infected cells. The E2 transcriptional activation protein ("the E2 protein") is a trans-acting factor that activates transcription through specific binding to cis-acting E2 enhancer sequences (i.e., E2 binding sites) in viral DNA (E. J. Androphy et al., "Bovine Papillomavirus E2 Trans-Activating Gene Product Binds to Specific Sites in Papillomavirus DNA", Nature, 324, pp. 70-73 (1987)). The 410 amino acid papillomavirus E2 protein has been shown to induce promoter expression in a classical enhancer mechanism (B. A. Spalholz et al., "trans-activation of a Bovine Papilloma Virus Transcriptional Regulatory Element by the E2 Gene Product", Cell, 42, pp. 183-91 (1985)). As with other transcription factors, the functions of E2 protein appear to be localized to discrete modular domains (I. Giri and M. Yaniv, "Structural and Mutational Analysis of E2 Trans-Activating Proteins of Papillomaviruses Reveals Three Distinct Functional Domains", EMBO J., 7, pp. 2823-29 (1988)).
Papillomavirus infections are non-lytic in their natural hosts, indicating that transcription and replication of the papillomavirus are tightly controlled. An upstream regulatory region ("URR") is found immediately 5' to the early genes of BPV and other papillomaviruses. The URR contains cis-acting regulatory signals, including an origin of DNA replication and several promoters that function in early gene transcription. The URR also contains enhancer elements that activate transcription from the URR promoters and heterologous promoters (Sousa et al., supra).
The E2 enhancer elements are conditional, in that they stimulate transcription only when activated by a protein encoded by a papillomavirus E2 open reading frame ("ORF"). Gene products from the E2 ORF include the full-length transcriptional activator E2 protein and at least two truncated versions of the E2 protein in BPV1 that function as transcriptional repressors. Transcriptional activation and repression of viral genes by E2 gene products constitute critical regulatory circuits in papillomavirus gene expression and DNA replication. E2 genes and DNA binding sites for E2 gene products appear to be characteristic of all papillomaviruses, although placement of the binding sites may vary Id.
Transcriptional regulation by the E2 protein depends on its direct binding to the nucleotide sequence 5'ACC(G)NNNN(C)GGT3' which is found within cis-acting E2 enhancer elements in all papillomaviruses (Androphy et al., supra; Dartmann et al., "The Nucleotide Sequence and Genome Organization of Human Papilloma Virus Type 11", Virology, 151, pp. 124-30 (1986); H. Hirochika et al., "Enhancers and Trans-Acting E2 Transcriptional Factors of Papillomaviruses", J. Virol., 61, pp. 2599-606 (1987); P. Hawley-Nelson et al., "The Specific DNA Recognition Sequence of the Bovine Papillomavirus E2 Protein is an E2-Dependent Enhancer", EMBO J., 7, pp. 525-31 (1988); A. A. McBride et al., "The Carboxy-Terminal Domain Shared by the Bovine Papillomavirus E2 Transactivator and Repressor Proteins Contains a Specific DNA Binding Activity", EMBO J.. 7, pp. 533-39 (1988)). In that sequence, N represents any nucleotide; X is any nucleotide--but is usually G, and Y represents any nucleotide--but is usually C. E2 binding sites appear to be positioned in close proximity to the viral promoters, with seventeen E2 binding sites being present throughout the bovine papillomavirus genome (R. Li et al., "Specific Recognition Nucleotides and their DNA Context Determine the Affinity of E2 Protein for 17 Binding Sites in the BPV-1 Genome", Genes Dev., 3, pp. 510-26 (1989)). in the URR's of all papillomaviruses, as well as in other sites near promoters throughout the viral genome.
E2 binding sites may function as an element in viral DNA replication, as well as a classical transcriptional enhancer element. E2-mediated DNA binding, therefore, is essential for the natural life cycle of papillomaviruses.
European patent application 302,758 refers to the use of modified forms of E2 protein that bind to, and block, E2 binding sites on papillomavirus DNA without resulting in trans-activation. That application also refers to repression of E2 activation through the use of DNA fragments that mimic E2 binding sites, and thus bind with E2 trans-activators, making them unavailable for binding to E2 sites on the viral DNA.
E2 protein also binds the papillomavirus replication protein known as E1. It has been proposed that when an E2/E1 complex binds to an E2 binding site, replication of the viral genome occurs (M. Botchan et al., International Papillomavirus Workshop, Heidelberg, Germany (May 1990); Mohr et al., "Targeting the E1 Replication Protein to the Papillomavirus Origin of Replication by Complex Formation with the E2 Transactivator", Science, 250, pp. 1654-99 (1990)).
Full-length E2 transcriptional activator polypeptides (monomers) have a molecular weight of about 50 kD. Although amino acid sequence homology among E2 proteins of various papillomaviruses is low (ca. 35%), the E2 proteins share conserved motifs that constitute unique structural domains having distinct functions (Giri and Yaniv, supra).
The C-terminal domain of the E2 polypeptide is responsible for recognition of E2 binding sites on viral DNA. The N-terminal domain of the E2 polypeptide is responsible for transcriptional activation following binding of the protein to viral DNA (A. A. McBride et al., "E2 Polypeptides Encoded by Bovine Papillomavirus Type 1 Form Dimers Through The Common Carboxyl-Terminal Domain: trans-Activation is Mediated by the Conserved Amino Terminal Domain", Proc. Natl. Acad. Sci. USA, 86, pp. 510-14 (1989)). The E2 protein binds to viral DNA in vivo only in the form of a pre-existing homodimer Id. Dimeric E2 proteins exert control of papillomavirus promoters by directly binding to an inverted repeat that has been found in all such viruses.
In bovine papillomavirus models, and in some human papillomaviruses, at least two N-terminally truncated E2 proteins occur naturally and act as native repressors. It has been experimentally confirmed in vitro that truncated forms of E2 proteins which retain their ability to bind DNA but do not trans-activate, are competitive inhibitors of trans-activation-competent E2 polypeptides (P. F. Lambert et al., "A Transcriptional Repressor Encoded By BPV-1 Shares A Common Carboxy-Terminal Domain With The E2 Transactivator", Cell, 50, pp. 69-78 (1987); A. Stenlund and M. R. Botchan, "The E2 Trans-Activator Can Act as a Repressor by Interfering with a Cellular Transcription Factor", Genes Dev., 4, pp. 123-36 (1990); J. Choe et al., "Bovine Papillomavirus Type 1 Encodes Two Forms of a Transcriptional Repressor: Structural and Functional Analysis of New Viral cDNAs", J. Virol., 63, pp. 1743-55 (1989)). That inhibition has never been definitively attributed to competition for DNA binding sites, for E2 polypeptides in the dimerization process, or for both. It has been suggested that transcriptional repression occurs through direct competition with the native full-length, i.e., transcriptionally active E2 protein at the DNA binding site. PCT patent application W089/12461 refers to peptide inhibitors of viral gene expression and viral replication. Those inhibitors are said to bind to trans-activator binding sites in viral DNA, thus blocking normal binding of native trans-activating proteins to those sites. And it has been suggested that formation of non-functional protein complexes could also prevent E2 activation of transcription (P. F. Lambert et al., supra).
Although it is known that papillomavirus E2 protein is the sequence-specific DNA binding protein that coordinates papillomavirus transcription, the structures of its DNA binding and dimerization motifs have never been determined. Both the DNA binding activity and the dimerization signal of the papillomavirus E2 trans-activation protein reside in the carboxy terminal 100 amino acids of the protein (McBride et al., supra). The C-terminal I00, 125 or 249 amino acids of E2 protein (each of which lacks trans-activation activity) all repress E2-dependent gene expression (T. Haugen et al., "Sequence-Specific and General Transcriptional Activation by the Bovine Papillomavirus-1 E2 Trans-Activator Require an N-Terminal Amphipathic Helix-Containing E2 Domain", EMBO J., 7, pp. 4245-53 (1988)). Although the capacity for E2 dimerization, as well as the capacity for site-specific DNA binding, are known to reside in the C-terminal domain of the E2 polypeptide, the amino acid region within that domain responsible for E2 dimerization has not been identified (Giri and Yaniv, supra). To date, the dimerization function of the E2 polypeptide has not been separated from its DNA-binding function. Accordingly, repressors that inhibit papillomavirus transcription and replication by interfering with dimerization of native full-length E 2 polypeptides have remained unknown.